Re: Why does a persons personality remain fairly consistent?

Hi Angela,

You ask an interesting question - and it is a question that scientists have 
been studying and debating for many years.  For some time it has been known 
that personality traits are relatively enduring qualities that (for the most 
part) are stable across time and across different situations.
However,  20-30 years ago, there was a debate among psychologists (the 
scientists that usually study personality) about whether personalities even 
existed.  Some people believed (and some still do) that our personalities are 
simply a function of the current situation - however, most scientists now 
believe that personality and personality traits are a combination of 
situational influences and biological factors (genes).  

I would have to agree and say that our personality and related traits are 
clearly a function of both our biology as well as our experiences.  Moreover, 
personality is malleable.  There are numerous reports that discuss the effects 
of psychological and physiological trauma on personality.  One of the most 
famous accidents was that of Phineas Gage (for more information see, http://www.sekhmet.org/~malice/
gage.html) who was struck by an iron rod that went through his skull.  He 
survived, however, he suffered a drastic change in personality as a result.  
However, I don't think you were asking about brain trauma ;-)   

As for whether people can change their personality on a whim - there certainly 
are everyday occurrences of this - usually with someone who has decided to turn 
over a new leaf (so to speak).  Many individuals who are struggling with 
difficulties (like drug abuse or alcoholism) often demonstrate some personality 
changes as they 'kick the habit' - however you need to question whether the 
drug they were using was causing some personality changes either directly or 
indirectly.
Scientists often use behavioral genetics to study personality.  

In the next few years the Human Genome Project (HGP for short) is going to come 
to fruition after sequencing all the genes in our DNA 
(http://www.ornl.gov/hgmis/about.html).  Behavioral genetics (BG for short) is 
the study of how genes and the environment interact to produce different 
behaviors.  One difficulty in studying this is teasing apart what the genes 
contribute to behavior, as well as what the environment contributes to 
behavior.  As you no doubt understand, a good science experiment only 
manipulates one variable at a time.  Thus, a good behavioral genetic study will 
keep either the genes constant, while manipulating the environment - or the 
converse.  Only recently has this become commonplace in the science labs.  

For much of this century, we have studied personality differences using Twin 
and Adoption studies.  Twin studies are observations of identical (monozygotic; 
MZ) or fraternal (dizygotic; DZ) twins.  What you probably remember from your 
biology classes is that MZ twins share all the same genes.  MZ twins are exact 
copies of each other (developing from the same fertilized egg), while DZ twins 
are basically just normal siblings who happened to be born at the same time (2 
fertilized eggs developing independently).  If you observe the behavior of many 
twins as they grow up, you can then compare the MZ and DZ twins on any number 
of personality traits.  If you see differences between MZ and DZ twins, then 
probably the genes had something to do with it.  We come to this conclusion 
because their environments were pretty much the same, but they differ on how 
much genetic information they share - MZ twins share ALL their genes, whereas 
DZ twins only share about half their genes).  Thus using twin studies, we can 
keep the environment the same (for the most part) while examining differences 
in genes.  Likewise, using Adoption studies, we can examine MZ twins who were 
adopted early in life by two different families.  This type of design permits 
us to keep the genes the same, while looking at different environmental 
influences.  As you might imagine, not many MZ twins adopted by different 
families exist, so the actual numbers of these individuals who we can study are 
small, however there are very significant findings.

Some recent studies have reported that personality traits like extraversion, 
neuroticism, agreeableness and conscientiousness are fairly well correlated 
between identical (MZ) twins - usually showing a correlation around r = 0.50.  
In contrast, fraternal (DZ) twins usually only end up with a correlation of 
about r = 0.20 for these same traits.

I hope this helps shed some light on your question.

Josh Rodefer, Ph.D.
Harvard Medical School